Cardiac output, the volumetric rate at which blood is pumped through the heart, is most often determined clinically by injecting a bolus of chilled saline or glucose solution into the heart through a catheter. A thermistor inserted in the blood at a point downstream of the heart as the chilled injectate/blood mixture is pumped from the heart, is used to determine a temperature--time washout curve; the area under this curve provides an indication of cardiac output. Although this thermo-dilution method can give an indication of cardiac output at the time the procedure is performed, it cannot be used for continuously monitoring cardiac output. The frequency with which the procedure is performed is limited by its adverse effects on a patient, including the dilution of the patient's blood that occurs each time the chilled fluid is injected. In addition, the procedure poses an infection hazard to medical staff from blood contact, and to the patient from contaminated injectate fluid or syringes.
An analogous method for measuring cardiac output involves the injection of a heated fluid into the heart; however, the same limitations on the frequency with which the measurement can be performed exist, whether the injectate is heated or chilled. Alternatively, blood in the heart can be chilled or heated by a heat transfer process using a temperature conditioned fluid that is circulated down one lumen within the catheter and returned back through another lumen. The principal advantages of using such a non-injectate heat transfer process to change the temperature of blood are that the blood is not diluted, and the temperature differential between the heat exchanger and the blood is reduced, compared to the differential temperature between an injectate fluid and blood in the typical thermal dilution method. U.S. Pat. No. 4,819,655 discloses an injectateless method and apparatus for determining cardiac output in this fashion.
Another technique for changing the temperature of blood circulating through the heart in order to determine cardiac output uses an electrical resistive heater that is disposed on the catheter and heated by an electrical current that is carried by conductors that run through one or more lumens in the catheter. A constant average power dissipation is typically maintained in the resistive heater, thereby enabling cardiac output to be determined as a simple function of the power dissipated and the temperature rise of blood measured downstream of the resistive heater. Several patents issued to H. Khalil, including U.S. Pat. Nos. 3,359,974, 4,217,910, and 4,240,441, disclose various catheters and monitoring systems for carrying out this procedure.
A disadvantage in maintaining a constant power dissipation in the resistive heater used in the preceding technique results from variations in the surface temperature of the heater as the rate of flow of blood past the heater changes. At relatively low rates of flow, the surface temperature of the catheter around the resistive heater can rise to a level at which damage to blood cells can occur. In addition, as the catheter is initially inserted into a patient's vascular system, care must be taken to turn off the electrical current used to heat the resistive heater, since the absence of a cooling blood flow can cause the resistive heater to become hot enough to burn when it is outside the body. To avoid damaging the blood or burning the patient in this manner, the maximum power dissipated in the heating element is severely limited. The temperature increase in blood flowing past the resistive heater at higher rates of flow is thus minimal, and as a result, the temperature change of the blood measured downstream is relatively low. This condition produces a poor signal-to-noise ratio at high volumetric flow rates, since the signal indicating temperature rise of the blood downstream of the heating element varies as the reciprocal of blood flow rate.
Previously Newbower (E. Trautman, R. Newbower, "The Development of Indicator Dilution Techniques", I.E.E.E. Trans. BME-31 No. 12 December 1984, pp. 800-807; R. Newbower et al., "Continuous Electronic Thermal Dilution Measurements," in Proc. 29th ACEMB, Boston, Mass., 1976) concluded that within the safety constraints noted above, heated catheter based thermal dilution could not be used in most patients clinically, because of poor signal-to-noise ratio with the limited safe maximum power.
Accordingly, it is an object of the present invention to provide a system for heating blood to determine cardiac output that avoids the limitations of prior art constant power dissipation systems, without creating a potential hazard of damaging blood cells by overheating or burning the patient. These and other objects and advantages of the present invention will be apparent by reference to the attached drawings and the Description of the Preferred Embodiments that follows.